The present invention relates to rotating rectifier assemblies for use in dynamo electric devices.
Dynamo electric devices such as self-excited, brushless AC generators typically utilize a rotating rectifier assembly to rectify the output of an exciter rotor and to feed the resulting DC power to a main generator rotor. Rectifier diodes enclosed in a DO-5 or DO-4 case have been traditionally used in such rotating rectifier assemblies with varying degrees of success.
The self-exciting brushless generators typically include three generators placed in tandem along a common rotating shaft. These three generators are retained within a common generator housing. When the machine is cooled by oil conduction, a portion of the interior of the housing is retained in a dry air-filled or gas-filled state surrounding the main field, exciter filed and the PMG field. Remaining portions of the housing and generator assembly are cooled by means of a fluid coolant circulation system.
A problem arises with the rectifier assemblies since they are constructed of semi-conductor material and therefore create a weak link in the generator system. The reason for the diodes being a weak link is that they undergo severe mechanical and thermal stresses during the operation of the generator. The parameters of the stresses are very difficult to overcome with known semi-conductor materials. While a variety of configurations and orientations of DO-5 and DO-4 type diodes have been employed, many of the thermal and mechanical problems persist. One type of generator has even used surface mounted devices (SMD) in an attempt to overcome such problems. However, due to the characteristics of the semi-conductor material used in the SMDs, the problems still persist.
With regard to the mechanical and thermal forces acting on the rectifier assembly, such forces are developed due to the nature of the operating conditions of the generator. For example, since the rectifier assembly is generally mounted close to the rotating shaft about which the rotors are positioned, the rectifier assembly, rotating with the rotating shaft, encounters severe centrifugal forces. The thermal conditions also place stresses on the materials of the diode which push the limits of the performance parameters of the semi-conductor material.
FIG. 2 provides a hybrid view of two prior art rectifier assembly configurations. A generator 20 is shown in FIG. 2 which has a rotating shaft 22 with a central axis 24 extending therethrough. A parallel oriented rectifier assembly is shown in the lower portion of the figure. In the parallel oriented rectifier assembly, the diodes 28 are oriented with a minor axis parallel to the central axis 24. The parallel oriented rectifier assemblies experience a substantial degree of centrifugal force due to the distance between the diode and the central axis. In extremely high speed generators, ordinary diodes are unable withstand the high centrifugal forces resulting in such a generator.
One attempt to overcome the problem of the high centrifugal forces, was to mount the diodes perpendicular to the central axis, thereby compressing the diode material and preventing structural fatigue due to high centrifugal forces. While a degree of success was achieved by compressing the diode material, a substantial amount of space was utilized in the perpendicular orientation.
Another problem encountered with the prior art rectifier assemblies as shown in FIG. 2, is that they are positioned in the air or gas enclosed space in which the fields operate, when used in a "conduction" cooled generator. As a result, these rectifier assemblies are exposed to high temperatures and are unable to benefit from the fluid coolant system utilized in other areas of the generator system.
While such assemblies may be acceptable for use in generators which rotate at relatively low speeds and have sufficient space available for the required mounting of electrical connections, these assemblies become a weak link in a generator which rotates at a relatively high speed. For example, a state-of-the-art generator, which may be used for aircraft, may rotate at speeds exceeding 30,000 rpm to meet the load requirements of the specific application.
More specifically, generators utilized in aircraft or other high technology dependent vehicles require very high power density generators. As a result, one of the most critical factors to achieving a high power density is to provide a high rotational speed. High rotational speed reduces the overall size of the generator. Two particular parameters which limit the rotational speed are the windage, a power loss caused by air friction on rotating bodies and centrifugal loading, a force incurred by a rotating body.